Secondary batteries are batteries which can be charged as well as discharged and thus repetitively used. A representative lithium secondary battery of the secondary batteries is operated by the principle in which lithium ions included in a cathode active material of a cathode are moved to an anode through an electrolyte and inserted into a layered structure of a anode active material (charging), and the lithium ions inserted into the layered structure of the anode active material return to the cathode (discharging). Such a lithium secondary battery is now commercially available to be used as a small power supply for a mobile phone, a notebook computer, etc., is also expected to be used as a large power supply for a hybrid car, etc., and is estimated that its demand will increased.
However, a composite metal oxide generally used as the cathode active material in the lithium secondary battery may be degraded by the reaction with the electrolyte. To solve this problem, in Korean Unexamined Patent Application Publication No. 2006-0119382, a cathode active material coated with a heterometal oxide is disclosed.